There is a need for low-powered radio frequency (“RF”) transmitters for such purposes as remote control or remote monitoring. For example, low-powered RF transmitters are used in garage door openers, automatic meter reading (AMR), remote keyless entry (RKE) and home automation. In order to accommodate this need, the Federal Communications Commission (“FCC”) has set aside certain radio frequency ranges (“bands”) known as “unlicensed ISM radio bands.” The maximum permitted power of transmitters operating in various ISM radio bands can vary. Exceeding the designated maximum power for a particular band is a violation of FCC rules in the U.S. and ETSI rules in Europe.
The need to set a maximum power for RF transmission has been addressed in the prior art. For example, attenuators have been used to reduce RF power. However, attenuators are expensive and wasteful of energy. This is particularly problematic with battery operated devices, such as remote controls.
In the past, cost constraints have dictated relatively inexpensive amplifiers to be used in battery-powered RF devices. For example, inexpensive class A amplifying devices have been widely used. Class A amplifiers operate over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. However, class A amplifiers are not very efficient and only have a theoretical maximum of 50% efficiency.
More recently, more efficient switching amplifiers have been used. Switching amplifiers are referred to as class E/F amplifiers and are highly efficient.
FIG. 1 illustrates a transmitter system 10 of the prior art which includes a high efficiency switching amplifier. The transmitter system 10 includes an integrated circuit (IC or “chip”) 12, designated by the broken lines, having the switching amplifier and a number of “off-chip” components such as a microcontroller (μC) 14, memory 16, RF choke (inductor) 18, resistor 20, bypass capacitor 22, attenuator 24, matching network 26 and a load RL (e.g. an antenna). The microcontroller 14 and memory 16 are used to digitally control the frequency of an output signal developed at an output 28 of the integrated circuit 12 and the resistor 20 is used to control the power of the output signal at output 28. The resistor 20 limits the maximum power that is delivered to the load (to meet FCC requirements) and the matching network 26 matches the impedance of the output signal to the load RL. In transmitter systems which do not use a resistor 20 to control the power, and the power output is fixed, an optional attenuator 24 can be coupled between the matching network 26 and load RL to prevent excessive power transmission. However, the optional attenuator 24 is not required if the power level is properly controlled by a resistor.
Integrated circuit 12 includes, among other components, a signal source 30, a buffer amplifier 32 and a switching-mode power amplifier 34. The frequency of the signal source 30 can be digitally controlled by microcontroller 14. For example, the signal source 30 can be a programmable phase lock loop (PLL). The buffer amplifier 32 provides a first stage of amplification and signal conditioning, and the switching-mode power amplifier 34 boosts the power which is developed at output 26. Integrated circuits similar to integrated circuit 12 have been developed by Maxim Integrated Products of Sunnyvale Calif. as, for example, product numbers MAX1472, MAX7044, MAX1479, MAX703x, MAX7057 and MAX7058.
The series connection of inductor 18 and resistor 20 between the output 28 of integrated circuit 12 and a voltage source VDD allows the power of the signal at output 28 to be controlled. This is accomplished by varying the resistance of resistor 20 to change the voltage level at the output 28 and thus the drain of power transistor (e.g. a MOSFET power transistor) 34. Due to the inductor 18, the voltage at the output 28 can vary up to a maximum of twice that of the voltage source (2×VDD). As the resistance of resistor 20 goes up, the voltage level (and therefore the power) at output 28 goes down, and vice versa. The bypass capacitor 22 shunts high frequency signals to ground.
While the described integrated circuit 12 has many advantages over prior art transmitters using less-efficient amplifiers, there is still room for improvement. For example, since the integrated circuit 12 allows the frequency at the output 28 to be varied, care must be taken to make sure the power output of the integrated circuit 12 does not exceed those proscribed by FCC regulations. This requires either an off-chip resistor (such as resistor 20) or an energy-wasteful attenuator 24. Furthermore, since the resistor 20 which determines the power output of the integrated circuit 12 is “off-chip” and is selected by a system integrator, the correct resistance value and the temperature characteristics of the system 10 must be empirically determined.
FIG. 2 is a graph illustrating a typical supply current and output power vs. external resistor curve of an RF transmitter system of FIG. 1. As noted above, a problem with this power control method is that an external resistor is required and that the gain cannot be changed ‘on the fly.’ In addition, the control characteristic is very non-linear with respect to power in dBm.
Amplifiers with a linear-in-dB control characteristic are desirable for a variety of RF system applications including gain control for receivers and level control for transmitters. Some circuit techniques have already been developed for linear-in-dB power control amplifiers. For example, there are those which utilize the exponential I-V relationship of bipolar junction transistors (see, for example, U.S. Pat. Nos. 5,200,655 and 5,684,431) and there are others that utilize the scaling properties of MOS devices (see, for example, U.S. Pat. Nos. 7,391,260 and 7,403,071). All of these techniques, however, are only operative with amplifiers that operate in the class A mode.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.